As this kind of combustion apparatus, there is known an apparatus from patent document 1 in which, in order to improve the heat resistivity of the combustion box, a water tube on an upstream side of a heat exchanger is disposed in a portion of the combustion box between the burner and the heat exchanger, as a cooling means for cooling the combustion box. Further, in this arrangement: a first packing is interposed between a combustion plate flange part on a circumference of the combustion plate which protrudes outward beyond an inner circumference of the body flange part and the body flange part; and a second packing is interposed between the combustion plate flange part and the connection flange part. In this arrangement, by the fastening operation the connection flange part to the body flange part, a combustion plate seal part disposed in the first packing will be compressed between the combustion plate flange part and the body flange part and, consequently, the sealing performance between the combustion plate flange part and the body flange part are secured. At the same time, a combustion box seal part disposed in the second packing will be compressed between the connection flange part and the combustion plate flange part and, consequently, the sealing performance between the connection flange part and the combustion plate flange part is secured.
By the way, the sheet-metal combustion plate will rise to a considerably high temperature at the time of combustion. Then, at the time of weak combustion at which the amount of air-fuel mixture in the burner body is small, the cooling function by the air-fuel mixture will be reduced and the heat is likely to be accumulated in the burner body. As a result, when the heat in the combustion plate is transmitted to the burner body, the burner body will rise to a considerably high temperature. As a consequence, an auxiliary part of the burner body, such as a check valve to be disposed in an inlet port to the burner body, will be subjected to a bad effect due to heat. In this case, it is conceivable to constitute the first packing in ceramic packing that is superior in thermal insulation performance so that the heat from the combustion plate becomes less likely to be transmitted to the burner body. This solution, however, becomes higher in cost.